Climate change is causing the Arctic Ocean sea ice to melt. When the polar ocean loses its sea ice cover, uptake of carbon dioxide increases, disrupting the food web in the water, according to a study in the journal Science co-authored by researchers from the University of Gothenburg .
By comparing data from a long list of Arctic expeditions, the researchers were able to see how the pH of the ocean north of Alaska and Siberia declined rapidly. In recent decades, the rate of acidification in the Arctic Ocean has been 3 to 4 times faster than in other oceans. This is because more carbon dioxide is absorbed by seawater when the surface is in direct contact with the atmosphere without an ice barrier. In the past, sea ice prevented the sea water near the North Pole from becoming saturated with carbon dioxide.
“The time series of pH measurements made in the Arctic Ocean is long. The oldest come from an expedition in 1994, when the ice cap was extensive and thick, and measurements were taken in the channels between the pack ice. During the 2014 expedition, the icebreaker Oden was able to travel in open water halfway from Siberia to the North Pole,” says Leif Anderson, marine chemistry researcher at the University of Gothenburg and one of study authors.
The greatest acidification in cold oceans
Acidification measured over the past 30 years corresponds to approximately 0.1 on the pH scale. The researchers estimate that the pH value will drop another 0.3 by the next turn of the century if carbon dioxide levels continue to rise in the atmosphere as they are today. The impact will be greatest in the coldest oceans. Water temperature determines the amount of carbon dioxide that can be dissolved in the ocean. More fresh water from melted sea ice also results in a greater acidification effect through absorption of carbon dioxide from the atmosphere due to changes in seawater chemistry.
Strong currents in the Pacific Ocean push nutrient-rich water to the polar oceans and during the long summer days in the Arctic, a high rate of primary production by phytoplankton occurs in its ice-free waters. . Phytoplankton absorb a lot of carbon dioxide during photosynthesis, and when water flows further north under polar ice, it is undersaturated with carbon dioxide. But now that sea ice cover is less extensive during the summer, uptake of carbon dioxide by seawater can continue and the oceans become more acidic.
Sea butterflies can be affected
Researchers have already seen that acidification affects marine life in the polar oceans.
“Phytoplankton, which absorb carbon dioxide, benefit from climate change. For other species however, the news is not so good. Sea butterflies are a species of predatory sea snails that have aragonite shells which they form from calcium and carbonate ions. We have measured lower and lower levels of aragonite saturation in the ocean during our expeditions,” explains Leif Anderson.
Sea butterflies are a keystone species and an important staple food for many whales that normally migrate to the Arctic Ocean for food and fattening. But now there may be less food in the pantry.
The reduction in sea ice cover is an important factor in the acidification of the polar oceans, and another is the warming of the High Arctic tundra. Large amounts of organic carbon are released and transported into Arctic rivers, where they break down, among other things, into carbon dioxide, resulting in lower pH values.
“All of these factors are consequences of climate change, and the increase in open water in the Arctic amplifies their effects,” says Leif Anderson.
Article in Science: Climate change drives rapid decadal acidification of the Arctic Ocean from 1994 to 2020